1. Field of the Invention
The exemplary embodiments of the present invention relate to an oil deterioration detection apparatus.
2. Description of Related Art
There is an oil deterioration detection apparatus, which uses dipole electrodes to detect oil deterioration by a PH of oil. A reference electrode and a sensitive electrode are used as the dipole electrodes. The oil deterioration detection apparatus measures an electric potential difference between the reference electrode and the sensitive electrode.
The oil deterioration detection apparatus has an oil deterioration detection part and an electric circuit. The oil deterioration detection part includes an oil deterioration-detecting sensor. The oil deterioration-detecting sensor has the reference electrode and the sensitive electrode. The electric circuit includes an operational amplifier and so on that measures the electric potential difference between the reference electrode and the sensitive electrode.
The oil deterioration-detecting sensor is used as a signal output circuit that outputs a true value of the electric potential difference between the electrodes. The operational amplifier is used as an input circuit that inputs the true value of the electric potential difference.
When the oil is deteriorated, the output signal varies. As a result, the oil deterioration detection apparatus can evaluates the deterioration of the oil.
However, in the oil deterioration detection apparatus, even if the oil deterioration-detecting sensor itself malfunctions, it does not detect a malfunction condition of the sensor itself. This is because that the output signal from the oil deterioration-detecting sensor varies based on the deterioration of the oil, so that the deterioration detection apparatus merely evaluates that the oil is deteriorated when the output signal varies. In addition, the oil deterioration detection apparatus does not have a detection element that detects the malfunction condition of the sensor. Therefore, it is difficult for the oil deterioration detection apparatus to distinguish the deterioration of the oil and the malfunction of the sensor only from the output signal. As a result, the oil deterioration detection apparatus may misunderstand the condition of the oil if the sensor malfunctions.
In addition, if the sensor malfunctions due to a broken wire, it has a problem that the input signal of the amplifier cannot be defined within a predetermined voltage range of the amplifier. In such a situation, the input circuit (amplifier) may be broken, too.
By the way, JP-A-H08-008708 discloses an input protection circuit. The input protection circuit has a protection transistor that is connected to an input circuit. A control voltage is supplied to a gate of the protection transistor, and an input signal is inputted into a drain of the transistor. An output, which is subtracted the gate-source voltage from a power supply voltage, is outputted though a source of the transistor. According to the input protection circuit, a stable drive signal is supplied to the input circuit.
However, the input protection circuit is not used for the oil deterioration detection apparatus, and cannot apply to define the input signal of the amplifier of the oil deterioration detection apparatus when the electrodes malfunctions of the sensor due to a broken wire.